1. Field of the Invention
The present invention relates to subterranean earth boring drill bits and, more particularly, to superabrasive cutters or cutting elements for use primarily on drill bits of the rotary drag type.
2. State of the Art
Rotary drag-type drill bits are comprised of a bit body mounted to a shank for connection to a drill string and having an inner channel or plenum communicating with the shank for supplying drilling fluid to the face of the bit. The bit body carries a plurality of cutting elements. Each cutting element may be mounted directly on the bit body or on a carrier, such as a stud or post, that is received in a socket in the bit body, typically on the bit face and sometimes on the gage.
When industrial quality natural and synthetic diamonds were first used on rotary drag bits, they were typically embedded into a metal substrate of a cutting element or as freestanding cutters in the metal matrix of a bit body. The diamonds had to be substantially embedded so that the mechanical nature of their attachment to the bit would withstand the high and diversely-oriented forces experienced during the drilling process, thus limiting the exposure of the diamonds to cut the formation.
Later, advances in the commercial production of synthetic diamonds made it possible to process diamond particles into larger disc shapes. The discs, or diamond tables, were typically formed of a particulate combination of sintered polycrystalline diamond and cobalt carbide. These diamond tables were formed during high-temperature, high-pressure fabrication and simultaneously bonded to a cemented tungsten carbide substrate, producing a cutter having a substantially planar cutting face. These cutters, generally termed "PDC's," for polycrystalline diamond compacts, are affixed to the bit body in the manner described above.
The diamond tables of PDC's, however, are susceptible to high temperatures, causing them to be more fragile and wear at higher rates as the temperature of drilling increases. In addition, these diamond tables do not provide any substantial kerfing action within the lateral extent of the path of each individual cutter during the drilling process. Kerfing is a process of making laterally-adjacent cuts, so that failure of the uncut rock between adjacent cuts affects (reduces) the overall energy required for drilling the formation. Because a single-depth diamond table has a continuous cutting edge, no kerfing action within the cutter path occurs. A so-called "claw" cutter has been developed, exhibiting a structure with parallel diamond ridges extending from the continuous major plane of the diamond table into and interleaved with the material of the supporting WC substrate. However, the kerfing action demonstrated by such cutters, as disclosed in U.S. Pat. Nos. 4,784,023 and 5,120,327, is nominal at best.
In order to manufacture diamond cutting elements of improved hardness, abrasion resistance and temperature stability, manufacturers developed a sintered PDC element from which the metallic interstitial components, typically cobalt and the like, were leached or otherwise removed to form thermally stable PDC's, or TSP's. However, due to present fabrication techniques, in order to leach the synthetic sintered PDC and achieve the desired improved temperature stability, it is necessary that these diamond elements be limited in cross sectional size. Other technologies have evolved wherein the interstitial components are replaced with silicon, but practical size limitations still exist, and the presence of silicon precludes effective metallic coating of the TSP's for non-mechanical bonding thereof to a bit body.
In order to use these TSP elements and yet achieve a larger, desired size of the cutting element, some prior art cutters incorporated an array of TSP elements disposed within a metal matrix substrate. Thus, the exposed ends of the TSP elements provided, in effect, a multi-element diamond table with a surface area substantially equal to the surface area of the ends of the TSP elements.
The prior art cutters employing a plurality of arrayed TSP elements have several disadvantages. Because these individual TSP elements replace the PDC diamond table, any substrate material between the TSP elements wears at a much higher rate than would a continuous diamond table. On the other hand, as previously mentioned, continuous PDC diamond tables are more significantly affected by heat, and may wear at an accelerated rate during the drilling process. In addition, PDC diamond tables alone do not generally provide any substantial single-cutter kerfing action. Thus, it would be advantageous to provide a cutting element for use in subterranean earth boring drill bits which provides the advantages of a continuous diamond table in combination with a plurality of additional diamond cutting structures affording additional strength and stiffness to the cutter, enhanced heat transfer away from the diamond table, and a kerfing action within the lateral bounds of a single cutter path.